The (eg ⊗ eu) ⊗ Eg product Jahn-Teller effect in the neutral group-IV vacancy quantum bits in diamond

The product Jahn-Teller effect may occur for such coupled electron-phonon systems in solids where single electrons occupy double degenerate orbitals. We propose that the excited state of the neutral XV split-vacancy complex in diamond, where X and V labels a group-IV impurity atom of X= Si, Ge, Sn, Pb and the vacancy, respectively, is such a system with e(g) and e(u), double degenerate orbitals and E-g quasi-localized phonons. We develop and apply ab initio theory to quantify the strength of electron-phonon coupling for neutral XV complexes in diamond, and find a significant impact on the corresponding optical properties of these centers. Our results show good agreement with recent experimental data on the prospective SiV(0) quantum bit, and reveals the complex nature of the excited states of neutral XV color centers in diamond.